Synthesis and characterization of a new hydrogen bonded side chain liquid crystal block copolymer and investigation of electrical properties

Materials

All chemicals used were analytical grade: Bromo acetylbromide (Aldrich), poly(ethylene glycol) methyl ether (PEG 550) (Aldrich), 2-(dimethylamino) ethyl methacrylate (Aldrich), triethylamine (Aldrich), CuBr₂ (Aldrich), CuBr (Aldrich), N,N,N′,N″,N″-pentamethyl diethylenetriamine (PMDETA) (Aldrich), ethylenediamine tetraacetic acid (EDTA) (Aldrich), dimethyl sulfoxide (DMSO) (Merck), 4′-hydroxy-4-biphenylcarbonitrile (Merck), 8-chloro-1-octanol (Across), anhydrous K₂CO₃ (Fluka), NaOH pellets (Merck), N,N-dimethylformamide (DMF) (Merck), 1-methyl-2-pyrrolidone (NMP) (Merck), tetrahyrofuran (THF) (Aldrich), and ethanol (Merck) were used as received.

Instruments

Spectroscopic characterizations of the polymers were carried out by FT-IR (Thermo Scientific Nicolet) and NMR Agilent VNMRS 500 MHz Nuclear Magnetic Resonance.

LC behavior of the polymers has been investigated by polarized optical microscopy (POM) using Leica DM2550P equipped with a LTSE350 Liquid Crystal Prosystem TMS 94 Hot Stage. Samples have been inserted between two thin glass cover slips with a heating-cooling rate of 10 °C/min.

Transition temperature and phase transition enthalpies have been determined by Differantial scanning calorimeter (DSC) using Perkin-Elmer Pyris6 calorimeter at a heating rate of 10 °C per minute under nitrogen atmosphere. Dielectric properties of polymers have been characterized by an impedance analyzer (HP 4194A) in the frequency range of 100 Hz–15 MHz.

Preparation of poly(ethylene glycol) methyl ether based macroinitiator (MPEG-550)

Bromo acetylbromide (10.33 mmol, 0.90 mL) in 10 mL of dry THF was added dropwise to a stirring mixture of triethylamine (13.20 mmol, 2 mL) and poly(ethylene glycol) methyl ether (10 mmol, 5 g) in 40 mL of dry THF at 0 °C. The reaction was continued under stirring for 18 h at room temperature. The solution was filtered, the solvent was evaporated, and the PEG based macroinitiator was precipitated in cold diethyl ether. The macroinitiator was filtered and dried under vacuum.

The synthesis of 2-(diethylamino) ethyl methacrylate block poly(ethylene glycol) methyl ether copolymer by ATRP Method (HBC-PEG550)

CuBr₂ (0.0032 g, 0.014 mmol), PMDETA (0.060 mL, 0.287 mmol) were dispersed after ultrasonication in NMP (3.0 mL) for 5 min. 0.2473 g of PEG based initiator was added and the mixture was deoxygenated via nitrogen gas. Then CuBr (0.021 g, 0.15 mmol) and 2-(dimethylamino) ethyl methacrylate (DEAEMA) (4.0 mL, 23.73 mmol) was introduced into the flask under nitrogen flow. Copolymerization reaction was performed at 60 °C for 18 h. After reaction, the mixture was filtered and was interact with EDTA solution to remove copper catalyst. The product was dried under vacuum at room temperature for 24 h and the yield was 5.06 g.

¹H-NMR (550 MHz, CDCl₃): δ(ppm): 0.82–0.84 (d, 3H, –C–(CH₃)), 0.99–1.01 (t, 6H, N(–C–CH₃)₂), 1.75–1.84 (m, 2H, –(CH₂–C–)_n), 2.54 (m, 4H, N(–CH₂–C)₂), 2.67 (t, 2H, C–CH₂–N), 3.32 (s, 3H, CH₃OCH₂CH₂O–), 3.58–3.60 (m, 16H, –OCH₂–CH₂–O–), 3.96 (t, 2H, –CH₂CH₂OCO–).

Synthesis of 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8)

Three gram (15 mmol) of 4′-hydroxy-4-biphenylcarbonitrile was in 200 mL of DMSO in the presence of 2 g (14.5 mmol) of K₂CO₃ anhydrous as acid scavenger. 3.4 mL of (20 mmol) 8-chloro-1-octanol was added drop wise to a stirring mixture under nitrogen at 110 °C. The reaction mixture was heated at 110 °C for 3 h. After this process, the reaction mixture was added drop wise to 400 mL of 10 % NaOH solution at room temperature and filtered. The resultant compound was dried at 40 °C in vacuum. It was re-crystallized from ethanol. Product was obtained in the form of white crystals and dried under vacuum. (Yield: 52 %) IR ν(cm⁻¹): 2224(CN), 3354(OH). ¹H-NMR (550 MHz, CDCl₃): δ(ppm): 1.2–1.8 (m, 12H, –(CH₂)₆–), 3.6 (t, 2H, HO–CH₂), 3.9–4.0 (t, 2H, CH₂–OAr), 6.9–7.0 (m, 2H, aromatic), 7.4–7.5 (m, 2H, aromatic), 7.6–7.7 (m, 4H aromatic).

Preparation of hydrogen-bonded side chain liquid crystalline polymer with LC8

To form H-bond, as a H-bond donor, 0.91 g of 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) (2.81 mmol) and as a H-acceptor polymer, 0.1751 g of block copolymer were dissolved in the DMF in 1 : 1 ratio. Solvent was slowly evaporated at room temperature and resulting solid liquid crystalline polymer was dried under vacuum for 24 h.

Synthesis and characterization

A new PEG-containing liquid crystalline side chain block copolymer was prepared by molecular self-assembly processes via hydrogen bond formation between 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) as a hydrogen bond donor and poly(ethylene glycol)-b-poly (2-(diethylamino) ethyl methacrylate copolymer (BC-PEG550) as a hydrogen bond acceptor.

For this purpose, block copolymer (BC-PEG550) was prepared by ATRP as detailed in the experimental section (Scheme 1).

Scheme 1:

Synthetic route used to prepare the block copolymer BC-PEG550.

BC-PEG550 was used to synthesize hydrogen bonded side chain liquid crystalline polymer (HBC-PEG550) as shown in Scheme 2.

Scheme 2:

Hydrogen bond formation between LC8 and BC-PEG550.

The molecular structure of the block copolymer BC-PEG550 and the formation of H-bonds between liquid crystalline mesogen LC8 and BC-PEG550 was confirmed by using ¹HNMR and FT-IR spectroscopy, respectively.

In the ¹H-NMR spectrum of BC-PEG550 (Fig. 1), the triplet peaks at 3.96 ppm corresponds to the COOCH₂ groups, the quartet and triplet peaks at 2.54 ppm, 2.67 ppm and 0.84 ppm corresponds to the N–CH₂– and N–C–CH₃ groups in the poly 2-(diethylamino) ethyl methacrylate block of the copolymer. The singlet peak at 3.32 ppm and triplet peaks at 3.59–3.60 ppm originate from the CH₃O– group and CH₂O group in the PEG block of the copolymer.

Fig. 1:

¹H-NMR spectrum of BC-PEG550.

The IR spectra of the BC-PEG550 and its H-bonded complexes with LC8 (HBC-PEG550) shown in Fig. 2a and b, respectively, are compared in order to analyze the formation of H-bond. The stretching vibrations of the N–C bond of the tertiary amine group in the poly 2-(diethylamino) ethyl methacrylate block appeared 1240 cm⁻¹. Incorporation of LC8 in to the polymer matrix leads to a shift of the band to 1246 cm⁻¹, indicating complementary hydrogen bond formation between tertiary amine group and hydroxyl group.

Fig. 2:

FT-IR spectra of the BC-PEG550 (a) and HBC-PEG550 (b).

Thermal and liquid crystalline phase behaviors of LC8 and HBC-PEG550 have been investigated by DSC and POM. The DSC (second heating) thermograms for HBC-PEG550 is given in Fig. 3. HBC-PEG550 exhibits two endothermic peaks at 92.22 °C and 103.34 °C which were assigned as glass transition-nematic and nematic-iso transition, respectively.

Fig. 3:

DSC curves of HBC-PEG550 and mesophase structures obtained from POM.

The phase behavior temperature and associated enthalpy change (ΔH) of LC8 and HBC-PEG550 are listed in Table 1.

Investigation by POM shows that the HBC-PEG550 exhibits an enansiotropic nematic liquid-crystalline phase. The textures of the mesophase have been identified during heating and cooling from isotropic melt at 107 °C as being a Nematic mesophase at 86 °C which is compatible with the DSC results.

Dielectric spectroscopy is a technique capable of probing the molecular motion and electrical properties of polymeric materials. It involves the measurement of the response of dipoles on polymeric main or side chains to a sinusoidally varying voltage. Dielectric spectroscopy (DS) informs that interactions of electrons, atoms, dipoles and self interfacial depending on frequency in terms of property of polarizable. Dielectric mechanism can be explain some parameters such as, dielectric constant, impedance, and dissipation factor. Alternative current (AC) measurements is one of the most effective techniques for dielectrical characterization of material which is known as dielectric spectroscopy.

The complex dielectric constant ε is expressed in terms of the real ε′ and imaginary ε″ components, which represents the stored and dissipated energy components of the material, respectively. In the complex plane, the complex dielectric constant for liquid crystal can be expressed as [27],

ε(ω)=ε′(ω)+iε″(ω)

Angular frequency ω referred to by the radial frequency, ω=2πf, where f is the frequency (measured in hertz) [28].

The complex impedance Z(ω) is commonly used to separate the surface and bulk material properties. The complex impedance and its complex conjugate of Z(ω) for express the sample,

Z(ω)=Z′(ω)+iZ″(ω)Z∗(ω)=Z′(ω)−iZ″(ω)

respectively. In these equations, ω is the angular frequency, Z(ω) and Z^*(ω) are the complex impedance and its complex conjugate, respectively. Z′(ω) and Z″(ω) are the real part, and the imaginary part of the complex impedance, respectively [29].

The tan δ is given by tan δ=ε″ε′; where δ=90−φ and φ is the phase angle. Dissipation factor (tan δ) is also a typical dielectric parameter.

The real ε′ and imaginary ε″ dielectric constant values decrease with increasing of frequency.

The dielectric constant and dielectric loss variations against frequency for BC-PEG550 and HBC-PEG550 samples are given in a single graph as seen in Figs. 4 and 5, respectively. The ε′ and ε″ is decreasing with the frequency. The ε′ and ε″ have high values due to LC electrode polarization effects. Also dielectric strength Δε has been calculated from Fig. 4. In the dielectric spectroscopy, the dielectric relaxation strength Δε is denoted as;

Fig. 4:

The real part of dielectric constant for BC-PEG550 and HBC-PEG550.

Fig. 5:

The imaginary part of dielectric constant for BC-PEG550 and HBC-PEG550.

Δε=εs−ε∞

where, ε_∞ and ε_S are high and low frequency dielectric constants, respectively and their calculated values are given in Table 2.

Here, the dielectric strength (Δε) value of HBC-PEG550 is lower than BC-PEG550. This situation is related to the polarization effect due to the molecular orientation of the liquid crystals.

Frequency dependency of electrical impedance for BC-PEG550 and HBC-PEG550 can be shown from Fig. 6. Frequency evolution of the impedance in log-log graph for all samples have been shown in Fig. 6. The electrical impedance decrease exponentially linearly with increasing of frequency for BC-PEG550. The impedance values are linearly decreasing with frequency for HBC-PEG550.

Fig. 6:

Frequency evaluation of the impedance for BC-PEG550 and HBC-PEG550.

Frequency dependency of the AC dissipation factor/energy loss for BC-PEG550 and BC-HBC-PEG550 are shown in Fig. 7. According to dissipation factor plots, the BC-PEG550 shows the first relaxation times at low frequencies for and HBC-PEG550 shows the first relexation times at low frequencies. On the other hand, HBC-PEG550 shows the first relaxation times at low frequencies and second relaxation times at high frequencies.

Fig. 7:

Frequency dependency of the dissipation factor for BC-PEG550 and HBC-PEG550.